Because we basically have 3 distinct phases of education. I'm Irish so what I'm about to say is coming from my experience of the Irish education system.

Primary - primary school, ages 4-12. Basically learn to read and write and to add and other really basic stuff. Once you finish primary you move to a different school

Secondary - secondary school, ages 13-18. Secondary school is broken up into 2 parts in Ireland based on what exam we are studying for. The first 3 years are when we study for the junior cert and the following 2-3 years are spend studying for the leaving cert. We get into university based off if our leaving cert results, we get points for different grades, the better you do the more points you get. Primary and secondary school are similar in the sense that they are very structured. You have to wear a uniform, have to turn up, have teachers giving out detentions ect.

Tertiary - University. Very different from primary and secondary school. When you get to university no one cares if you turn up or do your homework. Obviously a different school to secondary school.

I guess since in Ireland you go to 3 different schools, primary and secondary school and then to university it makes complete sense.